Process for forming on the surface of a dielectric a thin layer, in which the potential fall is concentrated



' PROCESS FOR FORMING N THE SURFACE OF A DIELECTRIC A THIN WHICH THEJPOTENTIAL FALL CONCENTBATED Patented July 1932 UNITED ST TES PATENT OFFICE:

Damn .rormt, or LENINGRAD, union or soexams'r sovm'r nnrunncs, assmnon 'ro mnus'rnm ammo]: 001mm, or camsamea, msacnusmrs, A coa- P OBATIOH OI MASSACHUSETTS mm m 80 Drawing. Application filed January 12, 1987, Serial No. 160,772, and lnGermany August 8, 1926.

' Renewed June 1, 1989.

My invention relates to insulators 'and dielectrics and aims to make them resistant to extreme potentials. An object of my invention is to accomplish the above with a minimum of insulating or dielectric material.

My new and improved insulator or dielectric can withstand such extreme potentials for such a little thickness of material, that enormous uantities of energy may be stored therein. hus, if used as a dielectric in an electric condenser, the energy stored in the condenser as a whole, which is a function of the otential applied and the capacity of the con enser, may be many times what is now 15 possible for the same'size. Furthermore, if

in a condenser, the plates are allowed to come close together or separate, the attractive forces due to a variation of the electrostatic field, will vibrate the plates in accordance with the ,go fluctuation of the field. This phenomenon is well known and has been taken advanta e of to design electrostatic voltmeters, telep one 25 may be used, and increasing the capacity of the condensers, allows the generation of enormous attractive and repulsive forces in this manner. 7

Afurther desirable result secured by my invention is the possibility of having compara= tively little and cheap insulation do the work-and in a better way-of the very complicated and highly expensive insulation now used in cables for high voltage transmission of electricity. The very wasteful and extremely undesirable silent discharge is entirely eliminted.

Inasmuch as the design and manufacture of my improved insulation and dielectric, though extremely simple, depends in a great theory upon which it works, I shall first state what I consider to; be the drawbacks and causes of failure of present day insulation. Then I shallstate my theory for such failure and disclose my new and improved insulation and methods of making it. Although I have based much of my work uponthe ines delineated by my theoryJa-nd have obtained results inaccord with the same theory, yet it 1s only a theory, subject to future changes and patent protection desired is the results obtained thereunder.

My researches in the field of insulators and dielectrics convinced me that the real underlying cause of their-breakdown is the generation of many ions by impact with the consequent conduction of an electric current therethrough. Thus we will consider a conductor with a coat of ordinary insulation. It is well known that there exist a number of free ions in the air and in most substances under ordinary conditions. How they are generated is 'not generally known. The sun with its radiant ener of various wave lengths, X rays, and ra io-active substances in the earth, and other similar causes, is undoubtedl responsible for the existence of many 0 these free ions. The number is variable and depends upon such factors as time of day, season, weather, etc. If such a free ion happens to wander into an electric field, such as might be produced by the potential which is impressed on our conductor, this ion will be drawn so that it tends to approachan oppositely charged pole. Thus,

if the ion is a positivel charged'one, and the conductor is charge positively, the ion will be repelled from it and drawn toward the negatively charged pole which may be either another conductor or ground. As is well known, a charged particle which is in a potential field, starts from a position of rest, at which position, its'potential energy is a maximum and kinetic ener is a minimum, and is gradually accelerate toward the opposite pole. When this particle has fallen through the entire potential, its potential energy is now zero, while its kinetic energy is a maximum. In the case at hand, however, such a particle could hardly fall through the entire potential uninterruptedly. In our case there will undoubtedly be some free ions in the insulation material adjacent the conductor Many of these, due to the polarity ofthe conductor 'will be repelled from it. Such a particle will have its acceleration always perpendicular to equipotential surfaces. As may be understood, the steeper the potential gradient, the greater the acceleration on the particle. Such a particle will, therefore, be travelling at a high speed in a comparatively short distance. The course of this particle under ordinary conditions is beset with the molecules composing the substance or medium in which it happens to be; here the medium would be the insulation. The result is that this accelerated ions, instead of pursuing a straight and direct course, probably pursues a very tortuous course, bumping-into molecules, rebounding, getting up speed again, bumping again and so on. Above a certain critical potential gradient, called the ionizing potential gradient, such an ion would be accelerated sufficiently within a distance of between 1 and 5 microns, so that if it collided with a molecule, the force would be suificient to break up the molecule into separate and opositely charged ions. Each of these ions would, of course, be attracted towards the pole having an unlike charge. Furthermore such ions would act the same way as the original free ion and would create other ions. 1

This state of affairs may be shown by a very simple equation. If D is the actual thickness of the insulator on our conductor, and if X is the distance within which the ion gets up suflicient speed to cause an ionizing impact with a molecule, then Z, the number of collisions of one originally free ion, will be equal to after which collision, the number of free or detached ions present is doubled. It is readily apparent, therefore, that the number of ions increases exponentially. Thus the number of ions, N, formed from one original ion, equals 2 raised to the Zth power, where Z is the number of collisions as above.

As was stated above, X is a very small number as a rule, and since D is usually very much greater than X, Z will be very large, while N will be enormous. To take a concrete case, imagine that the insulation on our conductor is about one quarter of an inch thick. This corresponds to about 6% mm. Assuming 5 microns or .005 mm. as X, which is a probable maximum under ordinary conditions, then Z would come out about 1300. N would therefore, be equal to 2 raised to the 1300th power. This number is almost inconceivable, there being between four and five hundred figures. Our ordinary numbers, even to billions, form but a very small fraction of such a number. When it is considered .that this number of ions is formed by only one free ion in the first place, it will lie readily apparent that the grand total will be still greater when multiplied by the number of free io'ns. Such a flow of ions is equivalent to a heavy flow of current, and as a matter of fact, a short circuit occurs when such a state of affairs is reached.

With present types of insulation, the potential gradient must be ke t below the ionizing gradient to avoid the a ove effects. This imposes a maximum stress on such insulation beyond which it is impossible to go, if the insulation is to last any length of time. Since the gradient is merely the fall of potential through a unit distance, it can readily be seen that if very great potentials are to be insulated roperly, the thickness of insulation must e greatly increased in order that the potential gradient remain below the ionizing point. In my new and improved insulation, the entire fall of potential is taken up by a very thin polarized layer. I have experimentally proven that the gradient or drop of potential per unit distance was the same at all parts of the polarized layer. The gradient, therefore, depends upon the thickness of this layer. Between the limits of a little less than 1 micron and about 5 microns of thickness, the potential that can be safely insulated by this layer, is practically independent of the thickness of the layer. This latter fact is not true of present day insulation.

It is to be distinctly understood that it is only the polarizedlayer that really performs the function of an insulator. If such a layer be formed on glass of mica, the rest of the glass or mica, which is not polarized, has very 7 little potential drop across it, and may be considered as a means of support for the polarized layer. Hence such support may be as thick or as thin as desired. It is possible to have this layer on each side of the insulator or dielectric, or a series of these layers may be -obtained by superposing laminations of insulating material, each one having polarized layers thereon. In such a case it is essential that no continuous ionic path be provided whereby substantial impact ionization might occur. A composite insulator of this kind may be produced by having several laminae of linseed oil varnish, each lamina treated to obtain a polarized layer. It seems that the linseed oil varnish proper is sufiiciently conducting so that any ions that come to it are conducted away and thus no very great amount of cumulative ionization is possible on the other side of the linseed oil impressed 60,000 volts across it without causing a breakdown. Sucha potential corresponds to 3 x 10 to the 6th power volts per cm., whereas the maximum potential gradient for ordinary insulation is about 2 x to the 5th volts per cm., and atthat potential gradient it is dangerously close to the ionizmg point.

Using the above insulator in a condenser throu h which high frequency current was passe the condenser conducted 10 amperes at 10,000 volts. The loss angle of the insulator under these conditions was only 2. As is well known, an ideal dielectric has no loss angle at all and in an ideal condenser, the current leads the potential by exactly 90. In the condenser that I use, with the loss angle mentioned, the lead angle was cut down to 88, which is much better than can be obtained in condensers where the dielectric is a solid or a liquid.

My improved insulator may be used (1) As an insulating material for all kinds of cables applied either in the form of a band or with an intermediate layer of paper, wood, fabric directly on the conductor,

(2) As insulation for highpotential conductors, and as a protection against corona and other silent. discharge,

(3) As insulating material for all kinds of electric machines, as motors, dynamos, magnetos, and transformers,

(4) As a dielectric material for all kinds of condensers, either for high voltages or high frequency,

r (5) As a means to enable a lar e amount of energy to be accumulated in hlgh powered condensers,

(6) Asa means for obtaining great mechanical forces in a condenser with moyable plates. I

As was stated, I obtained a polarized layer on an ordinary insulator, this polarized layer taking practically the entire potential drop 4 across it. An examination of this polarized layer showed that it was extremely poor in free ions. Furthermore, the thickness of this layer is such that the few free ions which are I accelerated can only collide a very small number of times and thus only a few ions are created. This means that only an infinitesimal current passes. Hence, an almost perfect insulating medium is thus created.

One method of creating such a layer is to heat the insulator to a high temperature,

though not high enough to permanently damage it, and at the same time pass a direct cur- I rent of electricity through it. It has been foundthat when a direct potential is applied to the opposite sides of an insulator by means of electrodes of, appreciable area, there is initially a fairly large surge of current which is the so-called charging current. The value of current then drops down to a fairly low value which cor tinues to pass for a considerable length .of time. Igunas been found, however, that this residual rrent decreases more and more until finall a very low value is reached. This action as been found to exist in insulators entirely free of any extraneous current carriers, such as moisture or metallic particles. through it for some time under these conditions, the amount of current grows smaller and smaller, finally dropping to a very low value. When this stage is reached, the current is stopped and the substance is allowed to cool. This method results in the production of a polarized layer. As stated above, this polarized layer is extremely poor in free ions. Thus it will be noted that this process consists essentially in the removal of these free ions from the surface layer. Inasmuch as the current which passes must result in the removal of the free ions, it is obvious that it must be these free ions which conduct the current through thelayer, and therefore the removal of these ions is what produces the decrease in the conductivity of said layer after the current has been passing for a considerable length of time. It should be noted at this point that the potential applied must be in onedirection in order that the ions have an opportunity to gradually progress to the surface of the insulator where they may escape into the external circuit. In order to accomplish the removal of these ions, it is further obvious that the layer must be free of extraneous matter which would conduct the current, and therefore prevent this current from removing the free ions. Such extraneous matter may be, for exampde, moisture or metallic particles and the like. I have treated pieces of glass, mica colophony, thin layers of oil, benzol, linseed oil varnish, calcite, mother of earl, colloidal carbonic salts, aluminium oxides, phosphates and cements, etc. In all these cases, a polarized layer was formed andit was observed that with the thickness of this layer within the limits mentioned above, the breakdown potential was practically independent of the thickness of the insulator. The polarized layer, moveover, will persist even when the substance has cooled.

Another method of producing such a layer After a current has passed is by the action of air oxidizing the surface of the dielectric to be treated. Thus linseed oil' is polymerized and purified at about 250 C. and then provided with a cobalt or manganese siccative. This product is then superficially oxidized at about 200 C. In such a case, there appears to form a superficial layer of oxidized substance, which is ermanent, and\ which has the property of eing polarized without the necessity of assing a current of electricity through it. n all such cases, thin layers in the neighborhood'of .001 mm. were obtainedit is the polarized layer in all such cases that is of this dimension, and not necessarily the entire insulating mediumwhich successfully withstood a potential gradient of 10 volts per cm.

Lclaim:

1. An insulating body having its insulat ing strength concentrated in one or more thin layers or" a thickness of the order of about five microns or less.

2. An insulating body comprising a plurality of layers, each of said layers comprising an insulating body having a thin layer of increased dielectric strength thereon, said thin layer being of a thickness of the order of between 1 and 5 microns.

3. The method of making an insulator resistant to high potential which consists in preparing a layer of material of very low conductivity, said conductivity depending prac-' tically solely on the presence of free ions in said layer, heating said layer, and causing a current of electricity to go through it in one direction until the free ions are removed from a thin layer on the surface of said first-named layer, said removal being observed by the decrease of said current to a value considerably less than its initial value.

In witness whereof I aflix my signature.

ABRAHAM J OFFE. 

